Hybrid poplars are the result of cross-fertilization of different species of the genus Populus. These poplars grow quickly, between 5 and 8 feet per year, making them beneficial in the production of paper products, firewood and lumber. Fungal diseases can prove problematic for hybrid poplars. While some of these diseases may only cause unsightly foliage and stunted growth, others may cause the eventual death of the tree.
Hybrid poplars infected with leaf rust display yellow or orange spots on their leaves in early to mid-summer. By late summer, leaves often fall from the tree. Leaf rust fungi over-winter on poplar leaves and infect other nearby trees in early spring before spreading back to the poplar tree.
Continued exposure to this disease can weaken hybrid poplars against cold weather and other diseases. Repeated outbreaks will stunt the tree's growth and possibly cause the tree to die. Plant hybrid poplars at least 500 yards away from other trees to prevent the disease. Purchase fungicides from a garden center and apply to the trees as described on the product's label.
Septoria Leaf Spot
Outbreaks of the fungal disease septoria leaf spot occur during cool, wet weather. This fungus over-winters on fallen foliage and transfers to other plants via wind and splashing rain. Small, dark flecks appear on the foliage and soon turn into larger spots. In severe cases, the spots merge together and cause leaves to turn brown or yellow and drop from the tree. Septoria fungi often cause cankers to form on the trunk and branches of hybrid poplars.
Chemical products will not control this disease. Prune back infected foliage, cleaning the pruning tool between each cut. Dispose of the diseased foliage and clean up fallen leaves to prevent the disease from over-wintering.
Marssonina Leaf Spot
Symptoms of marssonina leaf spot disease are similar to those of septoria leaf spot and often affect hybrid poplars grown in coastal areas of the western United States. Small spots merge into larger spots and eventually cause defoliation of the tree. Remove diseased branches and clean up fallen foliage throughout the year.
Venturia Leaf and Shoot Blight
The spores of venturia leaf and shoot blight form in the spring during cool, wet weather. Leaf spots and blights appear on the leaves and shoots of trees infected with this disease and cause the shoots to turn black and die. Leaf and shoot blight most often attacks young hybrid poplars that have not yet grown resistant to the disease. Prune back and remove infected shoots. Water directly into the soil and avoid getting the foliage of young trees wet. Apply fungicides according to the directions on the product. Keep the surrounding areas clean of fallen shoots to prevent over-wintering of the disease.